Indian mustard (Brassica juncea (L.) Czern.) belongs in the Brassicaceae and is widely cultivated in Korea for the edible leaves. In May 2011, Indian mustard plants of cv. Cheong-Kyeoja, growing in polyethylene-film-covered greenhouses in Hwaseong, Korea, were observed to be affected by a powdery mildew. Symptoms appeared as circular to irregular white colonies, which subsequently showed abundant hyphal growth on both leaf surfaces. Severely infected plants were unmarketable due to leaf discoloration, and most were not harvested. Voucher specimens have been deposited in the Korea University Herbarium (KUS). Appressoria on the mycelium were well developed, lobed, solitary, or in opposite pairs. Conidiophores were cylindrical, 70 to 115 × 8 to 10 μm, and composed of 3 to 4 cells. Foot-cells of conidiophores were straight to substraight, cylindrical, and relatively short (20 to 30 μm long). Singly-produced conidia were oblong to cylindrical or oval, 27.5 to 50 × 14 to 17.5 μm with a length/width ratio of 1.8 to 3.6, with angular/rectangular wrinkling of outer walls, and lacked distinct fibrosin bodies. Germ tubes were produced in the perihilar position of conidia. No chasmothecia were found. These structures are typical of the powdery mildew Pseudoidium anamorph of the genus Erysiphe. The specific measurements and characteristics were consistent with previous records of Erysiphe cruciferarum Opiz ex L. Junell (1). To confirm the identification, the complete internal transcribed spacer (ITS) region of rDNA of KUS-F24819 was amplified with primers ITS5 and P3 (4), and sequenced directly. The resulting 462-bp sequence was deposited in GenBank (Accession No. KC862331). A GenBank BLAST search of the ITS sequence showed 100% identity (462/462 bp) with those of isolates of E. cruciferarum from B. oleracea var. acephala, B. rapa, and Arabidopsis thaliana (GU721075, EU140958, and FJ548627, respectively). Pathogenicity was confirmed through inoculation by gently dusting conidia onto leaves of five healthy, potted Indian mustard plants of the cv. Cheong-Kyeoja. Five non-inoculated plants served as a control treatment. Inoculated plants were isolated from non-inoculated plants in separate rooms in a greenhouse at 18 to 24°C. Inoculated plants developed symptoms after 7 days, whereas the control plants remained symptomless. The fungus present on the inoculated plants was identical morphologically to that originally observed on the diseased plants, fulfilling Koch's postulates. There are records of E. cruciferarum infecting B. juncea in Europe (Finland, Romania, Sweden, Switzerland), the former Soviet Union, Australia, South Africa, and Asia (India, Japan, and China) (2,3). To our knowledge, this is the first report of powdery mildew caused by E. cruciferarum on B. juncea in Korea. Occurrence of powdery mildew is a threat to quality and marketability of this crop, especially those grown in organic farming where chemical control options are limited.
References: (1) U. Braun and R. T. A. Cook. Taxonomic Manual of the Erysiphales (Powdery Mildews), CBS Biodiversity Series No. 11. CBS, Utrecht, 2012. (2) D. F. Farr and A. Y. Rossman. Fungal Databases, Syst. Mycol. Microbiol. Lab., Online publication. USDA ARS. Retrieved 20 March 2013. (3) P. Kaur et al. Plant Dis. 92:650, 2008. (4) S. Takamatsu et al. Mycol. Res. 113:117, 2009.